The present invention pertains to the use of xcex2-ketoesters and their enamines for stabilising thermoplastic polymers.
The use of xcex2-ketoesters for stabilising polymers, especially vinyl chloride polymers, is known in the art. EP-A-433230 discloses alkylene and alkylenoxy xcex2-ketoesters such as butanediol bis-benzoylacetate and trimethylolpropane tris-acetoacetate, as stabilisers for chlorine-containing polymers. Similarly, JP-A-6-65458 (Chem. Abstr. 121 (1994) 135609t) describes a combination of ethyleneglycol diacetoacetate, a metal alkanoate and a phenolic antioxidant to be used for thermal stabilisation and colour protection in PVC.
Alkanolamide xcex2-ketoalkanoates such as tris-acetoacetoxyethyl isocyanurate as stabilisers for chlorine-containing polymers are described in EP-A-22749. xcex2-Ketones constitute another group of useful stabilisers. An effective but expensive xcex2-ketone stabiliser is stearoyl benzoyl methane, commercially available as Rhodiastab 50(copyright).
Mixtures of stabilisers containing at least a disaccharide alcohol such as maltitol or isomaltitol and a zinc compound (zinc stearate) are disclosed in EP-A-677549. These mixtures result in improved heat stability of polyvinyl-chloride compositions.
EP-A-599478 discloses the use of acetoacetate esters and their enamines, including sorbitol acetoacetate and trimethylolpropane acetoacetate, as a coalescent in water-based acrylic polymers.
EP-A-114270 describes isosorbide mono-acetoacetate and isosorbide mono-xcex2-amino-crotonate as intermediates in the preparation of isosorbide dihydronicotinate derivatives as anti-hypertensive drugs.
It was found that specific xcex2-ketoesters and their enamines, which can be obtained from inexpensive natural raw materials, have improved stabilising capacities for use in thermoplastic polymers such as PVC. These xcex2-ketoesters not only improve the initial colour stability of polymers, but also increase the thermal stability of the polymers. They can be used in rigid polymers, and they are also effective in stabilising flexible polymers. They have an improved performance to cost ratio. The xcex2-ketoesters and their enamines to be used according to the invention are derived from alcohols or polyols which are natural products or can be obtained from natural products by reduction or chemical and fermentative processes. They are defined in the appending claims. It may be noted that although the xcex2-keto-esters and the corresponding amines are represented in the formulae 1 and 2 as diketo compounds and keto-enamines respectively, they can also exist in their tautomeric forms of keto-enols and ketoimines, respectively, as in formulae 1a and 2a given below. All these and other tautomeric forms are understood to be included in the formulae used.
A[(Oxe2x80x94CHR4xe2x80x94Zxe2x80x94CHR5)mxe2x80x94Oxe2x80x94COxe2x80x94CR1xe2x95x90C(OH)xe2x80x94R3]nxe2x80x83xe2x80x831a
A[(Oxe2x80x94CHR4xe2x80x94Zxe2x80x94CHR5)mxe2x80x94Oxe2x80x94COxe2x80x94CHR1xe2x80x94C(xe2x95x90NR2)xe2x80x94R3]nxe2x80x83xe2x80x832a
The group A in the ketoester and enamine thereof according to the invention may be substituted by an aryl group, in particular phenyl, benzyl or phenethyl, which is optionally substituted by hydroxy and/or C1-C4 alkyl. As an example, one or more free hydroxy groups of the polyol, i.e. hydroxyl groups not esterified with xcex2-keto acid, may be esterified with benzoic acid or with 3-(3,5-di-t-butyl-4-hydroxyphenyl)propanoic acid (antioxidant group), or two of such free hydroxyl groups may be acetalised with a substituted benzaldehyde.
Examples of the xcex2-ketoesters and enamines to be used according to the invention are ketoesters of lactic acid esters, i.e. xcex1-(xcex2-ketoacyloxy)propionic acid esters, such as ethyl, butyl, lauryl, stearyl, and especially phenyl, 3,5-di-t-butyl-4-hydroxybenzyl and 2,2,6,6-tetramethyl-4-piperidyl esters.
A very useful group of xcex2-ketoesters and enamines is derived from sugars, sugar alcohols, dehydrated sugar alcohols and oligosaccharides. These are polyols, which preferably contain more than one xcex2-ketoester or enamine groups. Suitable sugars comprise glucose, galactose, fructose, (iso)maltulose, leucrose, lactose, sucrose, glucose oligomers such as trehalose, maltose, isomaltose, cellobiose, and higher homologues, such as maltotriose and cyclodextrins, fructose oligomers (especially inulin derivatives) and mixed oligomers and mixtures of these sugars, as well as glycosides of reducing sugars. Oligosaccharides are understood as short-chain polysaccharides having an average degree of polymerisation up to 40 monosaccharide units. Suitable sugar alcohols include xylitol, arabinitol, sorbitol, mannitol, galactitol, lactitol, maltitol, isomaltitol, maltotriitol and the like, and dehydration products thereof such as sorbitan, galactitan, isosorbide and other dianhydroglycitols such as isomannide and isoidide. As explained above, part of the polyol hydroxy groups may be free or be esterified or etherified with other groups, such as aryl groups.
xcex2-Ketoesters and their enamines of sugar acids and their esters, i.e. where the xcex2-ketoacyl groups or xcex2-aminocrotonoyl groups are attached to the hydroxy groups of the sugar acids or their esters, are also effective stabilisers according to the invention. Suitable sugar acids include the glyconic, glycuronic and glycaric acids such as gluconic acid, lactobionic acid and the like, and acids which can be obtained by oxidation or fermentation processes from sugars, such as lactic acid, citric acid, malic acid, tartaric acid, gluconic acid, L-ascorbic acid and the like.
The ketoester groups may be aliphatic groups, such as acetoacetyl, pivaloylacetyl, stearoylacetyl or methoxycarbonyl-acetoacetyl (the monoester of 3-oxoglutaric acid), but they may also contain aryl groups, such as in benzoylacetyl and ring-substituted derivatives thereof. The enamines of the ketoesters such as xcex2-aminocrotonates and N-substituted xcex2-aminocrotonates, are equally suitable.
The xcex2-ketoesters can be prepared in a manner known per se, e.g. by reacting the alcohol or polyol, i.e. the hydroxyalkanoic ester, sugar, sugar acid or sugar alcohol, with a lower alkyl ester of the xcex2-ketoacid with or without a transesterification catalyst such as a tetra-alkoxytin. It was found that such a catalyst is not necessary for the compounds of the invention, and that the purity of the products is even higher when a catalyst is not used. The absence of a catalyst has as a further advantage that traces of water do not affect the transesterification efficiency. Instead of using a lower alkyl ester of the xcex2-keto acid, the synthesis can also be performed using a diketene (4-alkylidene-2-oxo-oxetane) with the appropriate alcohol or polyol or dianhydroglycitol. Prior to the reaction of the alcohol or polyol with the xcex2-ketoacid lower alkyl ester or diketene, it may reacted with an epoxide such as ethylene oxide, propylene oxide or glycidol, eventually resulting in a xcex2-ketoacid ester having one or more alkylenoxy groups interposed between the xcex2-ketoacyl group(s) and the alcohol or polyol residue. The xcex2-aminocrotonates can be prepared by simply reacting the xcex2-ketoester with ammonia or with an amine such as methylamine or ethanolamine.
The xcex2-ketoesters and their enamines can be used in polymer compounds in a manner known per se. The stabilisers can be mixed with other additives, such as impact modifiers for rigid formulations (for example chlorinated polyethylene or butadiene/styrene/(acrylonitrile) co- or ter-polymers), plasticisers for flexible formulations (for example phthalic esters such as dibutyl phthalate or dioctyl phthalate, aliphatic monobasic or dibasic esters such as butyl oleate, epoxidised soybean oil, dioctyl adipate), fillers, pigments, flow modifiers (for example acrylates), lubricants (for example calcium stearate, zinc stearate, fatty esters and amides), flame retardants (for example aluminium hydroxide, antimony trioxide), phosphites (for example triaryl phosphites or aryl-alkyl phosphites), antioxidants (for example hindered phenols), HALS (hindered amine light stabiliser) compounds, UV absorbers (for example benzophenones such as 2-hydroxy-4-methoxybenzophenone, benzotriazoles, salicylates), other keto esters and ketones such as N-phenyl-3-acetyl-2,4-pyrrolidinedione; other polyol co-stabilisers, such as pentaerythritol, tris(hydroxyethyl)isocyanurate and the like, may also be used at reduced levels. Co-stabilisers and other additives that can be used together with the xcex2-ketoesters of the invention are also mentioned in EP-A-677549.
Preferred combinations of the xcex2-ketoesters and their enamines for use as stabilisers include combinations with polyols such as those described above (including e.g. sorbitol, lactitol and inulin), but also non-sugar polyols such as tris(hydroxyethyl)isocyanurate, (di)pentaerythritol or trismethylolpropane, in a weight ratio of e.g. between 1:10 to 10:1, especially of 1:4 to 4:1. The combination of the xcex2-ketoesters/enamines and polyols, especially sugar alcohols like sorbitol was found to be particularly useful, since it provides an excellent heat stability and colour stability. Other preferred combinations are those with inorganic materials including oxides and mixed oxides of e.g. alkali and alkaline earth metals, aluminium, silicon, titanium, zinc, tin and lead, such as hydro-talcites, silicates, dawsonites, zinc oxide and zeolites, and salts of such metals, especially zinc carboxylate salts (zinc stearate, calcium stearate and the like), and other organo-metal compounds such as stannanes.
The xcex2-ketoesters and their enamines can be used for stabilising thermoplastic polymers. These can be e.g. polyethene, polypropene, polystyrene, a halogenated rubber, fluorine-containing polymers, such as poly(vinylidene fluoride), poly(vinylidene-chloride) and, especially, poly(vinyl chloride). The chlorinated polymers such as PVC, the non-vinyls (polyethene and polypropene) and halogenated rubbers are preferred according to the invention. The xcex2-ketoesters are used at a level of 0.001-5%, especially 0.01-1%, with respect to the thermoplastic polymer.
The heat stability of a polymer like PVC can be expressed in the heating time at a selected temperature (e.g. 200xc2x0 C.) until the polymer degrades as determined by the colour turning brown to black. The test can e.g. be performed in a Matthis oven using a 25 cm polymer strip which is stepwise moved from the oven.
Non-vinyl polymers as PE are investigated in multi-extrusion tests, after the first, third and fifth run, the colour properties and the melt flow index are measured. Polyolefines are very sensitive to UV-light, therefore UV-stabilising tests are carried out.
The colour properties can be expressed as the whiteness according to Berger (Wb (%)) and the Yellowness index (Yi (%)). Both can be determined e.g. using a Minolta Chromameter with a DP 301 data processor. The rating is done according to the CIE L-a-b system (CIE: Commission Internationale d""Eclairage). White/black (L), green/red (a) and yellow/blue (b) values are converted to the Wb and Yi values. For optimum performance, the heat stability and the whiteness should be as high as possible, and the yellowness index should be as low as possible.
The formulations can be processed into a shaped article by means of calendering, rotational moulding, spread coating, slush moulding, extrusion, injection moulding, blow-moulding or other conventional technique.
In the examples that follow, the synthesis of stabilisers according to the invention and their use in polymer compounds are illustrated. The compounds tested are the PVC compounds A-D obtained from premixed components given below.